1. Field of the Invention
This invention relates to a method for manufacturing a cutter element for a drill bit of the type used to drill earthen formations and, specifically, to a method for bonding a composite compact to the mounting stud which is used to mount the compact as a cutter element in a drill bit.
2. Description of the Prior Art
In the past, conventional rotary drill bits for drilling oil and gas wells have used cutting elements including steel teeth, steel teeth coated with tungsten carbide, compact inserts of cemented tungsten carbide, and natural diamonds set or molded in tungsten carbide. In more recent years, synthetic diamond compacts have been utilized as the cutting elements in drill bits.
Certain of the prior designs have utilized drill bit cutting elements which were formed by attaching a diamond composite compact having a cemented carbide substrate to a cemented carbide stud by brazing the composite compact to the stud. The stud was then mounted in any of a number of ways to the drill bit, as by mounting the stud in a hole provided in the drill bit exterior surface.
The term "composite compact" is understood by those skilled in the art to means a cluster compact which is bonded to a substrate material such as cemented tungsten carbide. The term "cluster compact" is generally understood to mean a cluster of abrasive particles such as synthetic diamonds or cubic boron nitride. The abrasive particles are bonded together either in a self-bonded relationship or by means of a suitable bonding medium disposed between the crystals, or by a combination of these means. The methods for making cluster compacts of the type under consideration are described, for instance, in U.S. Pat. No. 3,136,615, U.S. Pat. No. 3,233,988 and U.S. Pat. No. 3,609,818.
The method of bonding the cluster compact to the substrate to form the composite compact utilized in the invention is known to those skilled in the art. The bond can be formed either during or subsequent to the formation of the cluster compact. For a better understanding of the techniques utilized in forming the bond, the reader is referred to U.S. Pat. No. 3,743,489, U.S. Pat. No. 3,745,623 and U.S. Pat. No. 3,767,371.
It is customary in the industry to provide an elongated base or stud for supporting the commercially supplied composite compact in mounting the compact on the drill bit. While it is technically feasible to form an integral composite compact of adequate length, the commercially adapted method is to braze an additional length of cemented carbide to the carbide base of the composite compact, because of significant cost savings in the latter method.
The present invention is directed to that step in the manufacturing process involving the attachment of commercially supplied composite compact to the cemented carbide stud which, in turn, is used to mount the composite compact as a cutter element in a drill bit.
In the past, brazing techniques for attaching the composite compact to the mounting stud were limited because the diamond layer of the composite compact is thermally degraded at temperatures above about 700.degree. C. Cubic boron nitride composite compacts are also thermally degraded at temperatures above about 700.degree. C. As a result, a brazing filler was generally required which had a liquidus below 700.degree. C. However, such brazing fillers have been found to generally form braze joints of lower strength than braze fillers having higher liquidus temperatures.
In U.S. Pat. No. 4,319,707 to Knemeyer, issued Mar. 16, 1982, a method and apparatus for brazing a composite compact to a mounting stud are shown in which the stud substrate is bonded to the base layer of the composite compact with a filler metal which, to form a bond, requires the exposure of the surface to be bonded to a temperature greater than the degradation temperature of the diamond layer. The apparatus shown heats the base layer, filler metal and substrate to a temperature in excess of the degradation temperature of the diamond layer while maintaining the temperature of the diamond layer below the degradation temperature with a heat sink. The heat sink of the apparatus features a cooling head or punch which contacts the diamond surface of the composite compact during the brazing step to keep the temperature of the diamond layer below the temperature at which it would thermally degrade.
Since the diamond surface of the composite compact is rough, as produced, it must be ground smooth in order to obtain maximum contact with the cooling head during the subsequent brazing operation. In the past, it was necessary to utilize a diamond grinder in order to grind or lap the exposed diamond surface of the composite compact. Since diamonds are required to cut or grind diamonds, this step added to the time and expense in the manufacturing process.
The present invention has as its object a method for reducing the manufacturing expense associated with providing a smoothly ground surface on the diamond end of the composite compact.
The invention also has as its object a method of providing a smoothly ground surface on a diamond composite compact without the necessity of utilizing a diamond grinding or lapping tool.